The math stack exchange is a forum for people to ask and answer math questions, ranging from the very easy to the very complicated.
The Online Encyclopedia of Integer Sequences is a site that keeps track of sequences of integers. When you run across a sequence of integers and wonder what it means or if it's been studied before, this is the place to look.
Sage is a powerful mathematical package that is free, open-source, and runs online.
Wolfram|Alpha looks like a browser, but it's actually a powerful knowledge engine, capable of doing math up through calculus and much more, if you ask it nicely.